156 



ROOTS 



Algae are not uncommon in soils and they seem to mfluence 

 soil fertility. Since Algae have chlorophyll and thus differ from 

 Fungi and Bacteria in being able to manufacture carbohydrates, 

 the Algae of the soil may furnish some of the carbohydrates which 

 the Fungi and Bacteria need. There is considerable evidence 

 that some of the soil Bacteria are directly benefited by the pres- 

 ence of Algae. Certain Bacteria and Algae are known to live 

 intimately associated, the Algae furnishing the Bacteria with 

 carbohydrates, and the Bacteria furnishing the Algae with ni- 

 trates. It is possible that some of the Algae and Bacteria of the 



soil are so related. In some labora- 

 tory experiments with soils, nitrates 

 have been formed more rapidly by the 

 nitrate-forming soil Bacteria when Al- 

 gae were present thanwhen Algae were 

 excluded, thus showing that in these 

 instances the nitrogen-fixing Bacteria 

 of the soil worked more efficiently when 

 Algae were present. 



The most numerous of the soil or- 

 ganisms are the Bacteria, the smallest 

 of organisms, and best known in con- 

 nection with diseases where they are 

 commonly called germs. The soil con- 

 tains many kinds, and commonly the 

 number of individuals per gram of soil 

 ranges from two to fifteen million. 

 Like the Fungi and Algae, the Bacteria 

 live in the surface layer of soils where 

 humus and air are present. The Bac- 

 teria, like the Fungi, decompose organic matter, making it avail- 

 able for higher plants, and some add atmospheric nitrogen to 

 the soU. The ammonifying, nitrogen-fixing, and denitrifying Bac- 

 teria are very important groups of soil Bacteria. 



The ammonifying Bacteria, of which there are a number of 

 kinds, act upon the nitrogenous substances in organic matter, 

 forming ammonia as one of the decomposition products. The 

 nitrifying Bacteria oxidize the ammonia, thereby producing 

 nitrates in which form the nitrogen is absorbed by the roots of 

 higher plants. 



Fig. 137. — A Mycorrhizaon 

 a rootlet of the Beech. The 

 felt-Uke mass of mycelial 

 threads closely enwraps the 

 root tip,^ extending back to 

 beyond the hair zone and 

 spreading into the soil like 

 root hairs. After Frank. 



